The compression-expansion process is a technique, which is mostly utilized for wireless microphones and so on, wherein a transmitter has a compressor circuit (compressor) for compressing signal(s) to be transmitted in a level, while a receiver has an expander for expanding the compressed, transmitted signal level to the original signal level, for purposes of suppression in noises or leaked speeches.
Most of the wireless microphones obtain improvement in the S/N ratio by, for example, the emphasized signals as known. The emphasis has a pre-emphasis process where the transmitter transmits signal(s), while performing expansion of gain in the high frequency range so as to suppress the generated noises. Since the generated noises depend on higher modulation frequencies of the transmitted signals, the emphasis process has a de-emphasis process in which the receiver performs reduction in the gain of the transmitted signal(s) so that the received signal(s) become the original signal(s).
Most of the noise and leaked speech reducing processes between the transmitter and receiver are performed in a combination of the compression-expansion technique and the emphasis technique. In such the wireless microphone, the compression-expansion technique and the emphasis process are performed in two orders (described below) for tuning of the mutual frequencies having the changes.
In the first process of noise and leaked speech reduction (as shown in FIG. 2 (a)), the pre-emphasis of transmission signal(s) by a pre-emphasis circuit of the transmitter 10, and the de-emphasis of the reception signal(s) by a de-emphasis circuit of the receiver 20 are performed before the transmitted signal(s) being compressed through a compressor circuit 12, and after the received signal(s) being expanded through an expander circuit 21, respectively.
On the other hand, in the second process of noise and leaked speech reduction (as shown in FIG. 2 (b)), the pre-emphasis of the transmitted signal(s) by a pre-emphasis circuit of the transmitter 10, and the de-emphasis of the reception signals by a de-emphasis circuit of the receiver 20 are performed after the transmitted signals being compressed through a compressor circuit 12, and before the received signals being expanded through an expander circuit 21, respectively.
The foregoing first process has some problems. The input of the signal(s) into the compressor circuit 12 after expansion of the gain in high frequencies by the pre-emphasis circuit causes distortion or skewness of the sound(s) in high frequencies. In order to reduce the skewness of the sound(s) in high frequencies, the reduction in the signal level (compressibility) adversely affects the S/N ratio in low frequencies. For improvement in such the S/N ratio, when the transmitter runs with a battery, the consumed battery must be considered.
In the foregoing second process where the signals are pre-emphasized after compressed, the transmitter has no production of the distorted or skewed sound in high frequencies as given in the foregoing first process. However, the performance of the expansion of the received signals by the receiver will be limited. Hence, even though the received signals are de-emphasized by the de-emphasis circuit of the receiver before expanded by the expander circuit, such the de-emphasis circuit does not have efficiency of the noise reduction in the signals outputted from the expander circuit.